Accessing all subpages of a book in order to save each is burdensome.
The solution could a function for downloading the page zipped with it's all subpages linked.
Possible steps:
1. Function for determining determining the list of pages to be downloaded from the name of a given page.
2. Function for assembling these by generating them locally to a temporary folder.
3. Function for changing the links inside the pages into filenames of the subpages.
4. Function for zipping the temporary folder.
5. Initiating the download.
The problem is also seem to be discussed at http://en.wikibooks.org/wiki/Help_talk:Readers'_FAQ

Just two raw ideas:
1) Tools like Offline Explorer can be used to save selected pages of a web site (as a remedy).
2) I think what should be requested here, is an "extension" which allows saving the pages in an special format (say PDF).

Comment on the two raw ideas:
1) Yes, OE can be useful, but just in some cases...
2) Saving a page in PDF <i>can</i> be useless in this case (namely, if you don't expand the subpages (pages with a URL that has "/" after the main article's namespace).